1. Field of the Invention
This invention relates to a semiconductor device suitable for an SOI (silicon on insulator) structure and having an isolation structure in which elements are isolated from one another by trenches.
2. Description of the Related Art
JP-A-5-136436 discloses a technique related to a high withstand voltage semiconductor device including trenches for dielectric isolation. An oxide film is used as a dielectric layer. Referring to FIG. 32, a silicon layer 202 is disposed on a silicon substrate 200 via a buried silicon oxide film 201, and a silicon island 204 is formed to be surrounded by a silicon oxide film 203 for trench side walls. A semiconductor element such as a LDMOS (lateral double diffusion MOS) transistor or an IGBT (insulated gate bipolar transistor) is formed in the silicon island 204. The silicon oxide films 201, 203 electrically isolate the transistor, and simultaneously support part of a high voltage applied across a drain and the substrate. Electrodes 205 formed on the surface of the transistor are completely and electrically isolated from the silicon substrate 200 by the silicon oxide film 201.
When several power elements are respectively formed in several silicon islands 204 using the technique described above, the thus obtained semiconductor device is suitable for use as a drive circuit capable of providing a high voltage and several power steps. For example, the device can be applied to a drive circuit for flat panel displays such as electroluminescent (EL) displays and plasma displays.
However, when the potential of the silicon substrate 200 is in a floating state, the silicon oxide film 201 does not support any voltage between the drain and the substrate. As a result, it becomes difficult to increase the withstand voltage of the semiconductor device. If an electrode is formed on the back surface of the silicon substrate 200 for applying an electric potential to the substrate 200, a process for TAB mounting or COG (chip on glass) mounting becomes complicated, resulting in increased manufacturing cost and decreased reliability.